1. Field of the Invention
The present invention relates to a multi-carrier radio transmission system such as OFDM (Orthogonal Frequency Division Multiplexing) radio transmission system. In particular, the present invention relates to apparatus and method for receiving an OFDM signal.
2. Description of the Related Art
According to one of multi-carrier, that is, OFDM, information is transmitted using several sub-carriers orthogonal to each other. Under radio communication environment, amplitude and phase variations (distortion) occur in a received signal by Rayleigh fading and multi-path due to a change of relatively positional relation between transmitter and receiver. When synchronizing detection is made, the foregoing distortion generated in a radio communication channel must be estimated for every sub-carrier.
In a wireless LAN system, a preamble signal (known signal) is sent to a header of a transmission frame. The preamble signal is used, and thereby, distortion in the transmission channel is estimated (channel estimation).
However, it is insufficient to only calculate a channel estimation value from the preamble signal; in this case, accuracy is worse, and also, the receiving characteristic is reduced. For this reason, there has been so far proposed a method of improving the accuracy of the channel estimation value estimated from the preamble signal. According to the foregoing method, a channel estimation value (vector value) is calculated from the preamble signal for every sub-carrier. Thereafter, a vector average is taken with respect to several channel estimation values to carry out smoothing (see, for example, Document 1: JPN. PAT. APPLN. KOKAI Publication No. 2001-197032, and Document 2: JPN. PAT. APPLN. KOKAI Publication No. 2001-268048).
According to the channel estimation method described above, a vector average is taken to carry out smoothing between sub-carriers. However, the channel estimation method has a problem that the channel estimation accuracy is reduced if the transmission channel variation between sub-carriers is large. In other words, the transmission channel variation between sub-carriers is large; nevertheless, a vector average of several sub-carriers is taken. In this case, the averaged vector receives the influence of sub-carrier having large amplitude; as a result, the phase difference between sub-carriers becomes non-uniform.